I'll probably give it a go during the weekends, cheers guys.
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Yeah making solaris unbeatable is pretty hard actually unless you give him 6 pokemon on the top of my head the strategies to beat an overleveled pokemons are intimidate shufling, perish song, destiny bond, endeavor, sturdy + quick attack. Pretty hard to account for all of them.
One strategy i can think of is giving solaris a pokemon with a multi hit move to bypass sturdy and such, but you can still intimidate shuffle or use quick claw purloin captivate if its a special attacker. This strategy requires a lot of rng and consumables tho. Mandibuzz learns bone rush so you can give it that, but mandibuzz attack is garbage x.x. Marowak or lucario maybe. Theme wise marowak and lucario also fits on solaris's sand team.
Edit: On second thought, making it impossible is not really a good idea as it takes away the challenge essentialy and forces the player to lose, don't mind me rambling haha